The search for increasing the efficiency of turbomachines, in particular in the field of aviation, and for reducing the fuel consumption and polluting emissions of gas and non-burned residues has resulted in using fuel in near-stoichiometric proportions. That situation is accompanied by an increase in the temperature of the gas leaving the combustion chamber and going towards the turbine.
As a consequence, the materials used in the turbine must be adapted to such an increase in temperature by developing techniques for cooling the turbine blades (hollow blades) and/or by improving the abilities of such materials to withstand high temperatures. That second route, in combination with the use of superalloys based on nickel and/or cobalt, has produced several solutions including depositing a thermally insulating coating termed a thermal barrier.
That type of ceramic coating forming a thermal barrier can create a thermal gradient across the coating on a cooled part running under steady operating conditions, with a total amplitude which may exceed 200° C. for a coating that is about 150 μm (micrometers) thick. The operating temperature of the underlying metal forming the substrate for the coating is reduced by the same amplitude, which induces large increases in the volume of cooling air required, reduces the service life of the part, and increases the specific consumption of the turbine engine.
Of the coatings that are in use, mention can be made of a ceramic based on zirconia stabilized with yttrium oxide.
Clearly, in order to improve the thermal barrier properties, in particular those of binding with the substrate and/or of providing protection against oxidation of the metal of the substrate, a sub-layer can be provided between the substrate and the outer layer of the coating.
In particular, it is known to use a sub-layer made of a MCrAlY type alloy, M being a metal selected from nickel, cobalt, iron, and a mixture of said metals, which alloy consists of a gamma matrix of nickel-cobalt with, in solution, chromium containing β NiAl precipitates.
It is also known to use a sub-layer formed by one or more aluminides, in particular comprising a nickel aluminide optionally containing a metal selected from platinum, chromium, palladium, ruthenium, iridium, osmium, rhodium, or a mixture of said metals, and/or a reactive element selected from zirconium (Zr), hafnium (Hf), and yttrium (Y). As an example, a Ni(l-x)PtxAl type coating is used in which the platinum is inserted into a nickel matrix. The platinum is deposited electrolytically prior to the thermochemical aluminizing treatment.
Normally, ceramic coatings are deposited onto the part to be coated either using a projection technique (in particular plasma projection) or a physical vapor deposition technique, i.e. by evaporation (in particular EB-PVD or “Electron Beam Physical Vapor Deposition”, forming a coating deposited in a vacuum evaporation chamber under electron bombardment).
With a projected coating, a deposit of oxide based on zirconia is made using plasma projection type techniques, which results in the formation of a coating constituted by a stack of fused droplets which are then shock quenched, flattened, and stacked to form an imperfect densified deposit which is generally in the range 50 micrometers (μm) to 1 millimeter (mm) thick.
A coating which is deposited physically, in particular by evaporation under electron bombardment, produces a coating constituted by an assembly of small columns directed substantially perpendicularly to the surface to be coated, over a thickness in the range 20μm to 600 μm. Advantageously, the space between the columns allows the coating to compensate effectively for thermomechanical stresses due, at operating temperatures, both to the expansion differential with the superalloy substrate and to centrifugal mechanical stresses due to rotation of the blades.
Further, to obtain a coating and/or a sub-layer of the coating, a step consisting in modifying the surface of the superalloy part is sometimes carried out by depositing a layer of platinum that is more than 10 μm thick, and then carrying out a thermal diffusion treatment.
Further still, the Applicant uses a thermochemical coating termed ClA formed by an aluminide coating modified with chromium and resulting from repeatedly carrying out two vapor deposition steps in succession: a first step of depositing a 2 μm to 6 μm thick layer of chromium followed by an aluminizing step.
Said coating is used as a coating to protect parts from oxidation or heat corrosion or, possibly, as a sub-layer for the thermal barrier.
Thus, parts are obtained with long service lives as regards high temperature thermal fatigue.
Conventionally, coatings forming thermal barriers thus create a thermal conductivity discontinuity between the outer layer of the mechanical part, including said thermal barrier, and the substrate of said coating forming the constituent material of the part.
However, such coatings, whether of the thermal barrier type or which protect against heat corrosion or oxidation, are obtained by methods (projection and/or physical vapor deposition and/or electrolytically) which are complex and expensive.
Further, in the event of localized damage to such coatings, whether during manufacture or during operation, the coating is completely reconditioned since a local repair is not possible, thereby engendering other problems. Stripping operations are difficult as they result in a reduction in the thickness of the substrate and an increase in the size of openings, with a corresponding reduction in the service life of the coated parts. After stripping, the coating manufacturing steps are recommenced over the entire surface of the part, and as a result the healthy zones are unnecessarily subjected to risky removal and reconditioning operations. Furthermore, a quantity of precious metal or metals (platinum, chromium, etc) is then lost.
It should also be noted that certain parts exhibit wear in particular zones, in particular at the leading edges and trailing edges of blades in the field of aviation, whether they be fan blades, compressor blades and/or turbine blades of a turbo engine.
In that case, cracks may be produced where the outer layer or even the sub-layer disappears locally, causing oxidation of the part. Such deterioration may require total repair of the part, which consists in removing the old coating, cleaning the part, reconstituting it, and rebuilding a new coating. Although they are very expensive, those operations are nevertheless carried out because the overall cost of the part is very high.
In some cases, said coating (sub-layer and outer layer in the case of a thermal barrier) is mechanically removed as well as reconditioned over a portion only of the part, but over an area which is generally quite large.